Over-sampled sigma-delta modulators are widely used in consumer audio and precision measurement devices. Signal processing is performed in the digital domain rather than in the analog domain, allowing for power savings and performance improvements as semiconductor processes improve. The modulator samples the input signal at a much higher frequency and spreads noise over a wider frequency band. A digital filter then decimates and filters the over-sampled data to remove the noise from the signal band.
FIG. 1 is a graph of modulator noise rejection as a function of the order of the modulator. The performance of a high-order over-sampled sigma-delta modulator is a function of the over-sampling ratio (OSR) and the order (L) of the modulator. The order of the modulator is the number of integrators cascaded together in a loop. FIG. 1 shows that higher-order modulators have a better signal-to-noise ratio for a given over-sampled ratio, and can be used for Analog-Digital Converters of higher precision (more bits of resolution). However, higher-order modulators tend to be unstable since oscillations occur for larger input levels.
In a Multi-stage noise-Shaped (MASH) sigma-delta modulator, the outputs from several low-order modulator loops are combined by a filter, such as a digital transfer function filter. The low-order loops are stable and cascaded together. The first stage may be constructed from larger transistors and capacitors to reduce thermal and op amp noise. However, this larger first stage also consumes more power than later stages constructed from smaller transistors and capacitors. Thus the first stage tends to provide a higher resolution while consuming more power than later stages, which have lower performance and lower power consumption.
Portable systems such as smart phones may use two or more sigma-delta modulators may be required for different applications. For example, a high-resolution sigma-delta modulator may be needed for playback of high-quality music recordings such as from a MP3 player, but a low-resolution sigma-delta modulator is better for reconstructing voice messages from telephone calls. During telephone calls the portable device's battery is being drained by the Radio-Frequency (RF) transceivers, so a lower power draw from the sigma-delta modulator is desirable at that time.
What is desired is a sigma-delta modulator that is adaptable for different applications. A configurable sigma-delta modulator is desired that can be configured for low-power lower-resolution applications and re-configured for high-power, high-resolution applications. A cascaded sigma-delta modulator that can power-down integrator loops and reconfigure connections to the remaining integrator loops and inputs to a digital filter is desirable.